Field of the Invention
The present invention relates to a semiconductor module and more particularly to a bonding structure in wire connection inside the semiconductor module.
Description of the Background Art
In recent times, growing environmental regulations enhance demand for high-efficient and energy-saving semiconductor modules that are environmentally friendly. The semiconductor modules, which are used for industrial equipment, drive control devices of household appliances with motors, electric cars, vehicle-mounted control devices for hybrid cars, railway control devices, and photovoltaic power generation control devices, for example, are required to be compatible with high electric power. The semiconductor modules are used under high load conditions (high temperature conditions) particularly in the vehicle-mounted control devices and the railway control devices from the viewpoint of saving energy and suppressing conversion losses of electrical energy (increased Tj), whereby the semiconductor modules are required to operate with high efficiency and low losses even under the high temperature conditions. Specifically, the previous normal operating temperature has been 150° C. or less at Tj=125° C., but the semiconductor modules are assumed to operate under the high temperature conditions of 200° C. or more at Tj=175° C. in the future.
To suppress switching losses under the above-mentioned high temperature conditions to achieve low losses and high efficiency at the high temperature state, the material and the structure of the semiconductor modules are needed to be reconsidered. Particularly, the wire connection portion connected to the external electrode deteriorates most easily, so that high quality, high reliability, and long life of the wire connection portion are difficult to be achieved.
In a case of the conventional soldering material, for a semiconductor module in a sandwich structure having a semiconductor element sandwiched between metal plates, a back surface electrode and a front surface electrode of the semiconductor element are each subjected to soldering by heating. For this reason, if the front surface electrode of the semiconductor element is soldered after the back surface electrode of the semiconductor element is soldered, the heating upon soldering of the front surface electrode causes the solder of the back side of the semiconductor element to melt again. This progresses Ni erosion of the back surface coated with metals, resulting in separation of the semiconductor element.
There is a semiconductor module performing a wire connection using a sintered bonding material including metal nanoparticles in place of the soldering material (for example, see Japanese Patent Application Laid-Open No. 2007-214340).
However, even in the case of using the sintered bonding material, the back surface electrode and front surface electrode of the semiconductor element each requires a bonding step. The sintered bonding material is bonded from the longer thermal history under the higher temperature conditions than the soldering material, so that the thermal stress is generated on each component of the semiconductor module, resulting in distortion and warpage. The sintered bonding material has a structure in which the metal particles coated with the surface stabilizer are stably dispersed in the solvent, and the surface stabilizer (solvent) is vaporized by heating. Therefore, the solvent vaporized upon bonding of the back surface electrode adheres to the front surface electrode of the semiconductor element, and then the front surface electrode becomes contaminated, which prevents from securing the bonding quality in the bonding portions bonded to the front surface electrode.